The United States' top general estimates more than 100,000 Russian soldiers have been killed and wounded in Ukraine, adding Kyiv's armed forces "probably" have suffered a similar level of casualties.
The estimates could not be independently confirmed by Reuters.
Chairman of the Joint Chiefs of Staff General Mark Milley's remarks offer the highest US estimate of casualties to date in the almost nine-month-old conflict.
They come as Ukraine and Russia face a potential winter lull in fighting that experts say could offer an opportunity for some kind of negotiations.
Asked about prospects for diplomacy in Ukraine, Milley noted the early refusal to negotiate in World War I compounded human suffering and led to millions more casualties.
"So when there's an opportunity to negotiate, when peace can be achieved ... seize the moment," Milley told the Economic Club of New York.
Earlier on Wednesday, Russia announced its troops would withdraw from the west bank of the Dnipro River near the strategic southern Ukrainian city of Kherson in a significant setback for Moscow and potential turning point in the war.
Some experts say the latest setback for Moscow could allow Ukraine to negotiate from a position of strength while others argue Russia might use negotiations to buy time to reset and refit its forces for a renewed spring offensive.
Milley said the initial indicators suggested Russia was following through with its withdrawal from Kherson but cautioned it could take time to complete.
"It won't take them a day or two," he said.
"This is going to take them days and perhaps even weeks to pull those forces south of that river."
Milley said Russia probably had 20,000 to 30,000 troops north of the Dnipro river in that area.
The US and its NATO allies have stopped short of direct intervention in Ukraine but are arming, advising and enabling its military to defend Kyiv against Russia's invading armies.
Milley said the conflict so far had turned anywhere from 15 million to 30 million Ukrainians into refugees and killed probably 40,000 Ukrainian civilians.
"You're looking at well over 100,000 Russian soldiers killed and wounded (and) same thing probably on the Ukrainian side. A lot of human suffering," Milley said.
Despite the high casualty numbers, US officials say Moscow has been unable to achieve its objectives in Ukraine and have raised questions about how long Russia will be able to sustain an invasion that has also decimated much of its mechanised ground forces and sapped artillery stockpiles.